The present invention relates to an under sheet for a lithographic printing plate to prevent the deviation (i.e., the skid) of the position of a lithographic printing plate on the plate cylinder of a printer (i.e., a printing press).
Printing is generally performed in a lithographic printer by winding a lithographic printing plate around a plate cylinder to fix the printing plate mechanically.
As the support of a lithographic printing plate, materials such as a metal, a plastic film and a paper have been conventionally used. A lithographic printing plate using materials other than metals as a support is excellent in handling property but is defective in dimensional stability as compared with a lithographic printing plate using a metal as a support.
When a lithographic printing plate having a support at least the back surface of which is a material other than a metal is used in a lithographic printer, the grasp position to the front end of the plate cylinder is liable to be made worse since the support is soft. In such a case the accuracy in the longitudinal position (the accuracy along the circumferential direction of the plate cylinder) becomes worse, and a lithographic printing plate is shifted diagonally and fixed in certain circumstances. Further, there is a problem that a deformation occurs partially due to the friction with a plate cylinder during printing, for instance, and the positional accuracy to the printing paper is deteriorated.
Accordingly, the use of a lithographic printing plate having a support at least whose back surface is a material other than a metal is limited to printing of a small number of papers where no problem arises even if a register accuracy of a printed matter is low, and if such a lithographic printing plate is used in multicolor precise printing or in printing of a large number of papers with a large-sized printer, there are cases where the deviation of colors is caused.
On the other hand, a plate-making method and a printing method by CTP (computer to plate) which have been prevailed in recent years are advantageous in that the dimension of imaging (exposure) and the positional accuracy are excellent and the register in multicolor printing is easy as compared with conventional plate-making and printing methods (exposure process of a printing plate material is performed by contact exposure using a lith film).
However, when a lithographic printing plate having a support comprising a material other than a metal such as a plastic film or paper is used, the advantage of easiness of the register in multicolor printing of CTP cannot be put to practical use due to the above drawback of the printing plate.
Further, it is suggested in recent years to interpose a sheet having an initial modulus of elasticity of 29xc3x97108 Pa (300 kg/mm2) or less between a printing plate and a plate cylinder (as described in JP-A-11-20130 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)). This sheet comprises fine glass beads and the like adhered and fixed so that central line average roughness (Ra) becomes 2 or more.
However, as is described in JP-A-11-20130 that this sheet can be produced by adhering fine glass beads and the like densely and uniformly on the surface of a sheet-like material, a solution having fine particles dispersed therein in high concentration is required to form concavities and convexities on the surface of a sheet.
These kinds of fine particles are generally expensive and a large quantity of fine particles are necessary to adhere them densely and uniformly, which results in the increase of the production cost of a sheet. Moreover, not only it is difficult to disperse fine particles in a solution in high concentration but also dispersion in high concentration is liable to cause the agglomeration of particles in the solution leading to coarse particles, and there arises a problem that the coarse particles deteriorate printing quality.
An object of the present invention is to provide an under sheet for a lithographic printing plate which can prevent the deviation of the position between the lithographic printing plate and the under sheet before printing and can be produced inexpensively.
The above object of the present invention can be achieved by an under sheet for a lithographic printing plate which has an initial modulus of elasticity of 34xc3x97108 Pa or more, and has concavities and convexities having central line average roughness (Ra) of less than 2 xcexcm and Ra/Rz of 0.05 or more on the surface in contact with the back surface of the lithographic printing plate.
According to the present invention, the skid (i.e., the deviation) of a lithographic printing plate on a plate cylinder caused by the pressure during printing on a lithographic printer can be certainly prevented by an under sheet for a lithographic printing plate which has an initial modulus of elasticity of 34xc3x97108 Pa or more, and has concavities and convexities having central line average (surface) roughness (Ra) of less than 2 xcexcm and Ra(center line average roughness)/Rz(ten-point mean roughness) of 0. 05 or more on the surface in contact with the back surface of the lithographic printing plate. Ra and Rz can be measured based on JIS B 0601. The term xe2x80x9cInitial modulus of elasticityxe2x80x9d in the present invention means the modulus of elasticity in the state before the under sheet is used in printing.
An under sheet in the present invention is used by being interposed between a plate cylinder and a lithographic printing plate having a back surface comprising at least a material other than a metal.
The under sheet in the present invention has an initial modulus of elasticity of 34xc3x97108 Pa or more, preferably from 34xc3x97108 to 98xc3x97108 Pa.
As the support for an under sheet, e.g., a metal plate, a resin sheet, and a composite sheet of a metal and a resin are used, preferably a metal sheet such as an aluminum plate, a zinc plate, a titanium plate, and a stainless steel plate, a bimetallic sheet such as a copper-aluminum plate, a copper-stainless steel plate, and a chromium-copper plate, a trimetallic sheet such as a chromium-copper-aluminum plate, a chromium-lead-iron plate, and a chromium-copper-stainless steel plate, a resin sheet such as a PET sheet, a PE sheet, a PP sheet, a polyester sheet, a polyimide sheet, a polyamide sheet, and an acrylate resin sheet, and a metal-resin composite sheet such as an aluminum-PET sheet, an aluminum-PE sheet, an aluminum-polyester sheet, a titanium-PET sheet, and a titanium-PE sheet, and more preferably a metal sheet such as an aluminum plate and a stainless steel plate, a resin sheet such as a PET sheet and a PE sheet, and a metal-resin composite sheet such as an aluminum-PET sheet and an aluminum-polyester sheet can be exemplified.
The thickness of an under sheet is from 50 to 350 xcexcm, preferably from 75 to 300 xcexcm, and more preferably from 100 to 250 xcexcm.
The concavities and convexities are formed on the surface of the under sheet in contact with the back surface for the lithographic printing plate so that the surface of the under sheet has a central line average (surface) roughness (Ra) of less than 2 xcexcm, preferably from 0.5 to 1.95 xcexcm, and more preferably from 1 to 1.95 xcexcm, and Ra/Rz of 0.05 or more, preferably from 0. 05 to 1, and more preferably from 0. 05 to 0.16. The protrusions can be formed by inorganic fine particles, organic fine particles and organic-inorganic composite fine particles.
As the inorganic fine particles, e.g., a metallic powder, a metallic oxide, a metallic nitride, a metallic sulfide, a metallic carbide and composite compounds of these compounds can be exemplified, preferably a metallic oxide and a metallic sulfide, and more preferably a metallic oxide such as glass, SiO2, TiO2, ZnO, Fe2O3, ZrO2 and SnO2, and a metallic sulfide such as ZnS and Cus can be exemplified.
As the organic fine particles, e.g., synthetic resin particles, natural high molecular weight particles can be exemplified, preferably an acrylate resin, polyethylene, polypropylene, polyethylene oxide, polypropylene oxide, polyethyleneimine, polystyrene, polyurethane, polyurea, polyester, polyamide, polyimide, carboxymethyl cellulose, gelatin, starch, chitin, and chitosan, and more preferably synthetic resin particles such as an acrylate resin, polyethylene, polypropylene, and polystyrene can be exemplified.
Composites comprising these organic and inorganic particles compounded in an arbitrary ratio are used as organic-inorganic composite fine particles.
These particles have an average particle size of from 1 to 100 xcexcm, preferably from 3 to 80 xcexcm, and more preferably from 5 to 50 xcexcm.
In the under sheet for a lithographic printing plate according to the present invention, big protrusions provided on the surface for forming the required concavities and convexities are sufficient at least to prevent the skid of a lithographic printing plate. The big protrusions pressed to the back surface of a lithographic printing plate in the concave state and bite the back surface of a lithographic printing plate. Accordingly, the skid of a lithographic printing plate on a plate cylinder caused by the function of the pressure during printing on a lithographic printer can be certainly inhibited.
In the present invention, concaving of the back surface of a lithographic printing plate may be performed during the process of interposing an under sheet between a plate cylinder and a lithographic printing plate with winding a plate cylinder up with a lithographic printing plate and an under sheet, or the back surface of a lithographic printing plate may not be concaved during the process of interposing an under sheet between a plate cylinder and a lithographic printing plate and may be concaved for the first time when pressure is applied after the process of interposing.
The kind of the lithographic printing plate for use in the present invention is not especially restricted and generally used PS plates, printing plates having a silver diffusible photosensitive layer, and electrophotographic printing plates may be used.
As the method of forming concavities and convexities on the surface of the support for an under sheet, a method of fixing the fine particles of glass and the like, which is harder than the material of the back surface of the support of a lithographic printing plate, on the surface of an under sheet having an initial modulus of elasticity of 34xc3x97108 Pa or more to form concavities and convexities can be exemplified.
As the specific examples of fixing fine particles on the surface of an under sheet to form concavities and convexities, a method of dispersing a coating solution comprising a binder having dispersed therein fine particles and coating the solution and drying, a method of preparing a binder film and then pushing fine particles into the binder film by mechanical pressure, and a method of preparing a binder film and electro-depositing fine particles can be exemplified.
A resin emulsion, a resin soluble in a solvent, an inorganic sol-gel and a resin-inorganic sol-gel composite are used as a binder, preferably a resin emulsion such as an acryl emulsion, a urethane emulsion, a polyethylene emulsion, a vinyl acetate emulsion, and a polyester emulsion; a resin soluble in a solvent such as an acrylate resin, polyethylene, vinyl acetate, polyurethane, polyester, and polyvinyl chloride; an inorganic sol-gel such as a silica sol-gel, a titanium sol-gel, and an aluminum sol-gel; a resin-inorganic sol-gel composite such as a polyvinyl pyrrolidone-silica composite sol-gel, a PVA-silica composite sol-gel, and a carboxymethyl cellulose-silica composite sol-gel; and more preferably a resin emulsion such as an acryl emulsion and a urethane emulsion; a resin soluble in a solvent such as an acrylate resin and polyethylene; an inorganic sol-gel such as silica sol-gel; and a resin-inorganic sol-gel composite such as a polyvinyl pyrrolidone-silica composite sol-gel and a PVA-silica composite sol-gel can be exemplified. These binders may be hardened by a self-crosslinking reaction and/or the introduction of crosslinking structure by using a crosslinking agent during film formation by drying.
As the method of fixing an under sheet on a plate cylinder, a method of providing an adhesive layer on the back surface of the support for an under sheet and using an adhesive or a pressure-sensitive adhesive such as a spray adhesive or a double-faced adhesive tape on the adhesive layer, a method of fixing the front end and the rear end of an under sheet by the clips provided on a plate cylinder and not providing an adhesive layer on an under sheet, or a method of combining these methods can be used.
The embodiment of the present invention will be described below.
When printing is performed with a lithographic printer, each lithographic printing plate is mounted on the plate cylinder of each printing unit via an under sheet. At this time, each under sheet is pressed to the back surface of the lithographic printing plate, thereby the protrusions on the surface of the under sheet concave the back surface of the lithographic printing plate to form concavities.
Thus, each under sheet adjusts the pressure by a rubber cylinder and an impression cylinder and at the same time prevents the positional deviation (i.e., positional skid) of the lithographic printing plate on the plate cylinder due to pressure.
In the next place, a method of preventing the positional deviation of a lithographic printing plate is described.
An under sheet having concavities and convexities of required contours on the surface thereof is interposed between a lithographic printing plate and a plate cylinder of each printing unit. At this time, the concavities and convexities on the surface of the under sheet are pressed to the back surface of each lithographic printing plate and the protrusions on the surface of each under sheet bite the back surface of each lithographic printing plate, thus the back surface of the lithographic printing plate is concaved concurrently with the concavities and convexities of the under sheet.